Sources of coherent radiation are used in a very wide variety of applications. In certain applications, such as spectroscopy, it is desirable to produce coherent radiation over a range of different wavelengths, typically by tuning a radiation source over a significant wavelength range. For example, for spectroscopic techniques using molecular rotations and vibrations, such as infrared absorption spectroscopy and Raman spectroscopy, coherent radiation needs to be provided at a specific wavelength necessary to excite a particular transition. This can then be used to detect the presence and concentration of a molecular species.
Tunable laser sources are readily available for the visible spectrum, but it is more difficult to find sources tunable over a significant range in the mid and long wavelength infrared spectrum (abbreviated to MWIR and LWIR respectively). Different definitions exist for these spectral regions, but one exemplary definition sets the MWIR region as the 3-8 μm wavelength band and the LWIR region as the 8-15 μm wavelength band. Optical parametric oscillators (OPO) are a particularly effective type of tunable source of coherent radiation in this wavelength region. An OPO is a system which converts an input pump laser into light beams at two different frequencies by passing it through a nonlinear optical crystal. These two beams are conventionally called the “signal” (the higher frequency beam and thus shorter wavelength beam) and the “idler” (the lower frequency beam and hence longer wavelength)—the sum of the frequencies of the output beams is the frequency of the input pump beam.
A well-established approach to providing a nonlinear optical crystal for use in an OPO for MWIR or LWIR operation is to use a periodically poled grating of birefringent material. Periodic poling involves the formation of a series of layers with alternately oriented ferroelectric domains. The thickness of these layers, and hence domains, is chosen to achieve quasi-phase-matching, which results in an efficient flow of energy from the pump frequency to the signal and idler frequencies. Periodically poled lithium niobate (PPLN) is a particularly preferred choice of nonlinear crystal for use in MWIR OPO operation. The skilled person will be aware of the numerous techniques (e.g. pulsed electric field, electron bombardment, thermal pulsing) available for creating the required periodic structure either during crystal growth or subsequently.
PPLN and similar periodically poled materials can be used to provide an output which can be tuned across a range by appropriate grating design. A preferred approach is to use a fan-out grating, as shown in FIG. 1. In a fan-out grating 1, in one dimension the domains are not uniformly thick but have a monotonically increasing thickness. If the pump laser beam 2 is scanned across the fan-out grating 1 in this dimension then the layer thickness passed through by the pump laser beam 2 varies monotonically (increases monotonically for the scanning direction shown in FIG. 1), and the effective operating wavelength of the OPO also varies monotonically. Using this approach, an OPO can be designed to operate over an extended frequency range (or wavelength range—reference to wavelengths and wavelength ranges will generally be made below, but these could equally be expressed as references to frequencies or frequency ranges). A grating of this type can be used inside or outside a resonant cavity of the OPO—an effective solution, particular for pulsed operation, is to use the grating inside the resonant cavity.
It is desirable to be able to use tunable sources of coherent radiation for real-time spectroscopic applications in extended spectral regions such as the MWIR and LWIR. One use that requires such a capability is hyperspectral imaging—for example, for the real-time detection of one or more molecular species at low concentration at a specific spatial location. In such a use, a conventional tunable source of the type described above may not be effective, as it will not be possible to investigate the necessary extended spectral range with sufficient resolution in a sufficiently short time for real-time detection.